Malaria is a disease that continues to have an impact in much of the developing world. This disease, which afflicts 200-300 million people, results in considerable morbidity (eg. fever and chills, malaise, anorexia, kidney disease and brain disease) and kills over one million children each year. The intracellular protozoa, Plasmodium falciparum, is the most virulent of human malarias and accounts for greater than 95% of malarial deaths. High levels of parasites in the bloodstream, seen especially in the P. falciparum infection, causes serious complications including severe hemolytic anemia, renal failure, and coma.
Minutes after entering a cutaneous blood capillary at the mosquito bite site (Vanderberg J P, Frevert U (2004) Intravital microscopy demonstrating antibody-mediated immobilization of Plasmodium berghei sporozoites injected into skin by mosquitoes. Int J Parasitol 34: 991-996), malaria sporozoites are arrested in the liver (Hollingdale M R (1985) Malaria and the liver. Hepatology 5: 327-335; Shin S C J, Vanderberg J P, Terzakis J A (1982) Direct infection of hepatocytes by sporozoites of Plasmodium berghei. J Protozool 29: 448-454). To infect a hepatocyte, their initial target cell in the mammalian host (FIG. 1), sporozoites must cross the layer of sinusoidal cells composed of specialized, highly fenestrated sinusoidal endothelia interspersed with Kupffer cells, the resident macrophages of the liver (Wisse E, Zanger R B, Charels K, Smissen P, McCuskey R S (1985) The liver sieve: considerations concerning the structure and function of endothelial fenestrae, the sinusoidal wall and the space of Disse. Hepatology 5: 683-692,4; Bouwens L, Wisse E (1992) The origin of Kupffer cells and their relationship to hepatocytes. In: Billiar T R, Curran R D, editors. Hepatocyte and Kupffer Cell Interactions. Boca Raton: CRC Press. pp. 3-21). Various in vivo, in vitro and, more recently, intravital studies have provided evidence that sporozoites cross the sinusoidal barrier by passing through Kupffer cells, which they selectively recognize and actively invade (Pradel G, Frevert U (2001) Plasmodium sporozoites actively enter and pass through Kupffer cells prior to hepatocyte invasion. Hepatology 33: 1154-1165; Pradel G, Garapaty S, Frevert U (2002) Proteoglycans mediate malaria sporozoite targeting to the liver. Mol Microbiol 45: 637-651; Frevert U, Engelmann S, Zougbédé S, Stange J, Ng B, et al. (2005) Intravital observation of Plasmodium berghei sporozoite infection of the liver. PLoS Biol 3: e192; Ishino T, Chinzei H, Yuda M (2004) A Plasmodium sporozoite protein with a membrane attack complex domain is required for breaching the liver sinusoidal cell layer prior to hepatocyte infection. Cell Microbiol 2: E4; Ishino T, Yano K, Chinzei Y, Yuda M (2004) Cell-passage activity is required for the malarial parasite to cross the liver sinusoidal cell layer. PLoS Biol 2: E4; Meis J F G M, Verhave J P, Brouwer A, Meuwissen J H E T (1985) Electron microscopic studies on the interaction of rat Kupffer cells and Plasmodium berghei sporozoites. Z Parasitenkd 71: 473-483; Meis J F G M, Verhave J P, Jap P H K, Meuwissen J H E T (1983) An ultrastructural study on the role of Kupffer cells in the process of infection by Plasmodium berghei sporozoites in rats. Parasitology 86: 231-242).
According to our current model (FIG. 1), the initial arrest of the parasites in the liver sinusoid is mediated by heparin-like heparan sulfate proteoglycans (HSPGs) of the liver (Cerami C, Frevert U, Sinnis P, Takacs B, Clavijo P, et al. (1992) The basolateral domain of the hepatocyte plasma membrane bears receptors for the circumsporozoite protein of Plasmodium falciparum sporozoites. Cell 70: 1021-1033; Frevert U, Sinnis P, Cerami C, Shreffler W, Takacs B, et al. (1993) Malaria circumsporozoite protein binds to heparan sulfate proteoglycans associated with the surface membrane of hepatocytes. (J Exp Med 177: 1287-1298). More particularly, sporozoites are initially arrested in the liver by binding with their two major surface proteins, the circumsporozoite protein (CSP) and the thrombospondin-related adhesive protein (TRAP), to unique extracellular matrix proteoglycans of the liver (Lyon M, Denkin J A, Gallagher J T (1994) Liver heparan sulfate structure. A novel molecular design. J Biol Chem 269: 11208-11215; Pradel G, Garapaty S, Frevert U (2002) Proteoglycans mediate malaria sporozoite targeting to the liver. Mol Microbiol 45: 637-651; Robson K J H, Frevert U, Reckmann I, Cowan G, Beier J, et al. (1995) Thrombospondin related adhesive protein (TRAP) of Plasmodium falciparum: expression during sporozoite ontogeny and binding to human hepatocytes. EMBO J 14: 3883-3894; Pradel G, Garapaty S, Frevert U (2004) Kupffer and stellate cell proteoglycans mediate malaria sporozoite targeting to the liver. Comp Hepatol 3 Suppl 1: S47; Gressner A M, Schäfer S (1989) Comparison of sulphated glycosaminoglycan and hyaluronate synthesis and secretion in cultured hepatocytes, fat storing cells, and Kupffer cells. J Clin Chem Clin Biochem 27: 141-149.) The parasites glide along the sinusoid and eventually recognize selected syndecans, small surface proteoglycans, on Kupffer cells, which they actively invade and traverse unharmed (Pradel G, Garapaty S, Frevert U (2002) Proteoglycans mediate malaria sporozoite targeting to the liver. Mol Microbiol 45: 637-651; Pradel G, Frevert U (2001) Plasmodium sporozoites actively enter and pass through Kupffer cells prior to hepatocyte invasion. Hepatology 33: 1154-1165; Frevert U, Engelmann S, Zougbédé S, Stange J, Ng B, et al. (2005) Intravital observation of Plasmodium berghei sporozoite infection of the liver. PLoS Biol 3: e192; Ishino T, Yano K, Chinzei Y, Yuda M (2004) Cell-passage activity is required for the malarial parasite to cross the liver sinusoidal cell layer. PLoS Biol 2: E4; Meis J F G M, Verhave J P, Brouwer A, Meuwissen J H E T (1985) Electron microscopic studies on the interaction of rat Kupffer cells and Plasmodium berghei sporozoites. Z Parasitenkd 71: 473-483; Meis J F G M, Verhave J P, Jap P H K, Meuwissen J H E T (1983) An ultrastructural study on the role of Kupffer cells in the process of infection by Plasmodium berghei sporozoites in rats. Parasitology 86: 231-242; Baer K, Roosevelt M, Van Rooijen N, Clarkson Jr. A B, Frevert U (2006) Kupffer cells are obligatory for Plasmodium sporozoite infection of the liver. Cell Microbiol in press; Frevert, U. et al. (2006), Nomadic or sessile: can Kupffer cells function as portals for malaria sporozoites to the liver? Cell. Microbiol. In Press; Frevert, U. et al., (2006), Penetrating biological barriers. Liver: Plasmodium Sporozoite Passage across the Sinusoidal Cell layer. In: Soldati D, Burleigh, B A, editors. Molecular Mechanisms of Parasite Invasion: Landes, In Press.). After exiting Kupffer cells towards the space of Disse, sporozoites traverse several hepatocytes before eventually settling down for development to thousands of erythrocytes-infective merozoites (Frevert U, Engelmann S, Zougbédé S, Stange J, Ng B, et al. (2005) Intravital observation of Plasmodium berghei sporozoite infection of the liver. PLoS Biol 3: e192; Mota M M, Pradel G, Vanderberg J P, Hafalla J C R, Frevert U, et al. (2001) Migration of Plasmodium sporozoites through cells before infection. Science 291: 141-144).
Liver heparan sulfate is unique in that its degree of sulfation approaches that of heparin, i.e. markedly higher than that of heparan sulfate from any other tissue (Lyon M, Denkin J A, Gallagher J T (1994) Liver heparan sulfate structure. A novel molecular design. J Biol Chem 269: 11208-11215). More specifically, sporozoites express two major surface proteins, the circumsporozoite protein (CSP) and the thrombospondin-related adhesive protein (TRAP), and these are thought to recognize extracellular matrix (ECM) proteoglycans inside the space of Disse (Pradel G, Garapaty S, Frevert U (2002) Proteoglycans mediate malaria sporozoite targeting to the liver. Mol Microbiol 45: 637-651; Robson K J H, Frevert U, Reckmann I, Cowan G, Beier J, et al. (1995) Thrombospondin related adhesive protein (TRAP) of Plasmodium falciparum: expression during sporozoite ontogeny and binding to human hepatocytes. EMBO J 14: 3883-3894; Pradel G, Garapaty S, Frevert U (2004) Kupffer and stellate cell proteoglycans mediate malaria sporozoite targeting to the liver. Comp Hepatol 3 Suppl 1: S47; Gressner A M, Schäfer S (1989) Comparison of sulphated glycosaminoglycan and hyaluronate synthesis and secretion in cultured hepatocytes, fat storing cells, and Kupffer cells. J Clin Chem Clin Biochem 27: 141-149). The large ECM proteoglycans are thought to protrude from the space of Disse through the endothelial sieve plates into sinusoidal lumen (Pradel G, Garapaty S, Frevert U (2004) Kupffer and stellate cell proteoglycans mediate malaria sporozoite targeting to the liver. Comp Hepatol 3 Suppl 1: S47), where they provide a basis for sporozoites to glide along the sinusoidal éndothelium. When the parasites encounter a Kupffer cell, CSP binds to selected chondroitin sulfate and heparan sulfate proteoglycans (syndecans) on the cell surface in a multivalent interaction that is thought to contribute to sporozoite adhesion to these macrophages (Pradel G, Garapaty S, Frevert U (2002) Proteoglycans mediate malaria sporozoite targeting to the liver. Mol Microbiol 45: 637-651). After exiting Kupffer cells towards the space of Disse (FIG. 1), sporozoites recognize small cell surface HSPGs on hepatocytes (Frevert U, Sinnis P, Cerami C, Shreffler W, Takacs B, et al. (1993) Malaria circumsporozoite protein binds to heparan sulfate proteoglycans associated with the surface membrane of hepatocytes. J Exp Med 177: 1287-1298; Pinzon-Ortiz C, Friedman J, Esko J, Sinnis P (2001) The binding of the circumsporozoite protein to cell surface heparan sulfate proteoglycans is required for Plasmodium sporozoite attachment to target cells. J Biol Chem 276: 26784-26791). This model is supported by direct observations of P. berghei sporozoite infection of the livers of live mice (Frevert U, Engelmann S, Zougbédé S, Stange J, Ng B, et al. (2005) Intravital observation of Plasmodium berghei sporozoite infection of the liver. PLoS Biol 3: e192).
CSP binding to the surface of mammalian cells (Shakibaei M, Frevert U (1996) Dual interaction of the malaria circumsporozoite protein with the low density lipoprotein receptor-related protein (LRP) and cell surface heparan sulfate. J Exp Med 184: 1699-1711; Ying P, Shakibaei M, Patankar M S, Clavijo P, Beavis R C, et al. (1997) The malaria circumsporozoite protein: interaction of the conserved regions I and II-plus with heparin-like oligosaccharides in heparan sulfate. Exp Parasitol 85: 168-182; Ancsin J B, Kisilevsky R (2004) A binding site for highly sulfated heparan sulfate is identified in the amino-terminus of the circumsporozoite protein: Significance for malarial sporozoite attachment to hepatocytes. J Biol Chem 279: 21824-21832; Rathore D, Kumar S, Lanar D E, McCutchan T F (2001) Disruption of disulfide linkages of the Plasmodium falciparum circumsporozoite protein: effects on cytotoxic and antibody responses in mice. Mol Biochem Parasitol 118: 75-82; Rathore D, McCutchan T F (2000) Heparin can regulate the binding of Plasmodium falciparum circumsporozoite protein. Mol Biochem Parasitol 108: 253-256; Rathore D, McCutchan T F, Garboczi D N, Toida T, Hernaiz M J, et al. (2001) Direct measurement of the interactions of glycosaminoglycans and a heparin decasaccharide with the malaria circumsporozoite protein. Biochemistry 40: 11518-11524) is mediated by a dual interaction with 1) syndecans, a family of small transmembrane proteoglycans that are expressed on almost all cell types (Bernfield M, Kokenyesi R, Kato M, Hinkes M T, Spring J, et al. (1992) Biology of the syndecans: a family of transmembrane heparan sulfate proteoglycans. AnnRevCell Biol 8: 365-393), and 2) the low density lipoprotein receptor-related protein (LRP-1), a multifunctional scavenger receptor that is predominantly expressed in the liver (Strickland D K, Kounnas M Z, Argraves W S (1995) LDL receptor-related protein: a multiligand receptor for lipoprotein and proteinase catabolism. FASEB J 9: 890-897; Strickland D K, Kounnas M Z, Williams S E, Argraves W S (1994) LDL receptor-related protein (LRP): a multiligand receptor. Fibrinolysis 8, Suppl.: 204-215; Herz J (1993) The LDL-receptor-related protein—portrait of a multifunctional receptor. Curr Opin Lipidol 4: 107-113). LRP-1, also known as the α2-macroglobulin receptor (α2MR) or CD91, is responsible for the clearance from the blood of a large number of molecules, including activated alpha-2-macroglobulin (α2M*), proteases and their complexes with inhibitors, matrix proteins, and growth factors, as well as small particles such as lipoprotein remnants (Strickland D K, Kounnas M Z, Argraves W S (1995) LDL receptor-related protein: a multiligand receptor for lipoprotein and proteinase catabolism. FASEB J 9: 890-897; Strickland D K, Kounnas M Z, Williams S E, Argraves W S (1994) LDL receptor-related protein (LRP): a multiligand receptor. Fibrinolysis 8, Suppl.: 204-215; Herz J (1993) The LDL-receptor-related protein—portrait of a multifunctional receptor. Curr Opin Lipidol 4: 107-113). Both syndecans and LRP-1 induce intracellular signaling cascades. Depending on the cytoplasmic domain of their various core proteins, syndecans are involved in distinct, but overlapping signal transduction cascades (Carey D J (1997) Syndecans: multifunctional cell-surface co-receptors. Biochem J 327: 1-16; Rapraeger A C (2000) Syndecan-regulated receptor signaling. JCell Biol 149: 995-997; Rapraeger A C (2001) Molecular interactions of syndecans during development. SemCell DevBiol 12: 107-116; Rapraeger A C, Ott V L (1998) Molecular interactions of the syndecan core proteins. Curr Op Cell Biol 10: 620-628; Woods A, Couchman J R (1998) Syndecans: synergistic activators of cell adhesion. Trends Cell Biol 8: 189-193; Woods A, Oh E-S, Couchman J R (1998) Syndecan proteoglycans and cell adhesion. Matrix Biology 17: 477-483; Zimmermann P, David G (1999) The syndecans, tuners of transmembrane signaling. FASEB J 13 (Suppl.): S91-S100). LRP-1 is directly or indirectly responsible for a large variety of signal transduction events. A direct role of LRP-1 in signal transduction is supported by the finding that receptor ligation leads to tyrosine and serine phosphorylation of its cytoplasmic domain (Li Y, van Kerkhof P, Marzolo M P, Strous G J, Bu G (2001) Identification of a major cyclic AMP-dependent protein kinase A phosphorylation site within the cytoplasmic tail of the low-density lipoprotein receptor-related protein: implication for receptor-mediated endocytosis. Mol Cell Biol 21: 1185-1195; Barnes H, Larsen B, Tyers M, van Der Geer P (2001) Tyrosine-phosphorylated low density lipoprotein receptor-related protein 1 (Lrp1) associates with the adaptor protein SHC in SRC-transformed cells. J Biol Chem 276: 19119-19125; van der Geer P (2002) Phosphorylation of LRP1: regulation of transport and signal transduction. Trends Cardiovasc Med 12: 160-165; Bu G, Sun Y, Schwartz A L, Holtzman D M (1998) Nerve growth factor induces rapid increases in functional cell surface low density lipoprotein receptor-related protein. J Biol Chem 273: 13359-13365) and that signaling adapter proteins such as Shc, Disabled, and FE65 associate with the NPXY motifs in the cytoplasmic domain (Barnes H, Larsen B, Tyers M, van Der Geer P (2001) Tyrosine-phosphorylated low density lipoprotein receptor-related protein 1 (Lrp1) associates with the adaptor protein SHC in SRC-transformed cells. J Biol Chem 276: 19119-19125, Gotthardt M, Trommsdorff M, Nevitt M F, Shelton J, Richardson J A, et al. (2000) Interactions of the low density lipoprotein receptor gene family with cytosolic adaptor and scaffold proteins suggest diverse biological functions in cellular communication and signal transduction. J Biol Chem 275: 25616-25624; Boucher P, Liu P, Gotthardt M, Hiesberger T, Anderson R G, et al. (2002) Platelet-derived growth factor mediates tyrosine phosphorylation of the cytoplasmic domain of the low density lipoprotein receptor-related protein in caveolae. J Biol Chem 277: 15507-15513; Bacskai B J, Xia M Q, Strickland D K, Rebeck G W, Hyman B T (2000) The endocytic receptor protein LRP also mediates neuronal calcium signaling via N-methyl-D-aspartate receptors. ProcNatlAcadSciUSA 97: 11551-11556; Trommsdorff M, Borg J P, Margolis B, Herz J (1998) Interaction of cytosolic adaptor proteins with neuronal apolipoprotein E receptors and the amyloid precursor protein. J Biol Chem 273: 33556-33560). Direct LRP-1 signaling can be inhibited by the receptor-associated protein (RAP) and occurs via the MEKK/JNK/cJun pathway (Lutz C, Nimpf J, Jenny M, Boecklinger K, Enzinger C, et al. (2002) Evidence of functional modulation of the MEKK/JNK/cJun signaling cascade by the low density lipoprotein receptor-related protein (LRP). J Biol Chem 277: 43143-43151; Schneider W J, Nimpf J (2003) LDL receptor relatives at the crossroad of endocytosis and signaling. Cell Mol Life Sci 60: 892-903).
A well-documented example for an indirect role of LRP-1 in signal transduction is its cooperation with the alpha-2-macroglobulin (α2M) signaling receptor (α2MSR) on peritoneal macrophages (Misra U K, Chu C T, Rubenstein D S, Gawdi D S, Pizzo S V (1993) Receptor-recognized a2-macroglobulin-methylamine elevates intracellular calcium, inositol phosphates and cyclic AMP in murine peritoneal macrophages. BiochemJ 290: 885-891; Misra U K, Chu C T-C, Gawdi G, Pizzo S V (1994) The relationship between low density lipoprotein-related/a2-macroglobulin (a2M) receptors and the newly described a2M signaling receptor. JBiolChem 269: 18303-18306; Misra U K, Chu C T-C, Gawdi G, Pizzo S V (1994) Evidence for second a2-macroglobulin receptor. JBiolChem 269: 12541-12547; Misra U K, Gawdi G, Pizzo S V (1999) Ligation of low-density lipoprotein receptor-related protein with antibodies elevates intracellular calcium and inositol 1,4,5-trisphosphate in macrophages. ArchBiochemBiophys 372: 238-247). The α2MSR, which binds activated α2M (α2M*) exclusively and with a much higher affinity than LRP-1, was recently identified as the heat shock protein Grp78 (Misra U K, Akabani G, Pizzo S V (2002) The role of cAMP-dependent signaling in receptor-recognized forms of alpha 2-macroglobulin-induced cellular proliferation. J Biol Chem 277: 36509-36520). Signaling through Grp78 is not inhibited by high molar excess of RAP, but it requires the presence of LRP-1 on the cell surface (Bacskai B J, Xia M Q, Strickland D K, Rebeck G W, Hyman B T (2000) The endocytic receptor protein LRP also mediates neuronal calcium signaling via N-methyl-D-aspartate receptors. ProcNatlAcadSciUSA 97: 11551-11556). Upon ligation with α2M*, the α2MSR activates a pertussis toxin-insensitive phospholipase C (PLC), which hydrolyses membrane phospholipids and generates the two second messengers inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG). IP3 causes the release of Ca++ from the endoplasmic reticulum, which triggers several Ca++-dependent signaling cascades. DAG activates protein kinase C (PKC), which causes phosphorylation-dependent signal transduction via p21Ras-dependent MAPK and phosphoinositol 3-kinase (PI3-kinase), leading to DNA synthesis and mitogenesis (Misra U K, Pizzo S V (1998) Ligation of the a2M signaling receptor with receptor-recognized forms of a2-macroglobulin initiates protein and DNA synthesis in macrophages: The effect of intracellular calcium. BiochimBiophysActa 1401: 121-128; Misra U K, Pizzo S V (1998) Ligation of the α2M signalling receptor elevates the levels of p21Ras-GTP in macrophages. CellSignal 10: 441-445). By signaling via phospholipase A2 (PLA2) activation, α2M* acts like known growth factors, thus explaining its mitogenic effect on macrophages (Misra U K, Gonzalez-Gronow M, Gawdi G, Hart J P, Johnson C E, et al. (2002) The role of Grp 78 in alpha 2-macroglobulin-induced signal transduction. Evidence from RNA interference that the low density lipoprotein receptor-related protein is associated with, but not necessary for, GRP 78-mediated signal transduction. J Biol Chem 277: 42082-42087). In addition, α2M* binding to α2MSR raises the intracellular concentration of cyclic adenosyl mono-phosphate (cAMP) followed by increased phosphorylation of MEK 1/2, ERK 1/2, p38MAPK, and JNK; these events culminate in cell proliferation by elevating the transcription factors nuclear factor κB (NFκB) and cAMP response element-binding protein (CREB) and expression of the proto-oncogenes c-fos and c-myc (Misra U K, Akabani G, Pizzo S V (2002) The role of cAMP-dependent signaling in receptor-recognized forms of alpha 2-macroglobulin-induced cellular proliferation. J Biol Chem 277: 36509-36520; Misra U K, Gonzalez-Gronow M, Gawdi G, Hart J P, Johnson C E, et al. (2002) The role of Grp 78 in alpha 2-macroglobulin-induced signal transduction. Evidence from RNA interference that the low density lipoprotein receptor-related protein is associated with, but not necessary for, GRP 78-mediated signal transduction. J Biol Chem 277: 42082-42087). The binding of other LRP-1 ligands such as lactoferrin, lipoprotein lipase, and Pseudomonas exotoxin A to macrophages also increases the intracellular concentration of Ca++, cAMP, and IP3, and activates PKA [Misra U K, Chu C T-C, Gawdi G, Pizzo S V (1994) The relationship between low density lipoprotein-related/a2-macroglobulin (a2M) receptors and the newly described α2M signaling receptor. JBiolChem 269: 18303-18306; Misra U K, Chu C T-C, Gawdi G, Pizzo S V (1994) Evidence for second α2-macroglobulin receptor. JBiolChem 269: 12541-12547; Misra U K, Akabani G, Pizzo S V (2002) The role of cAMP-dependent signaling in receptor-recognized forms of alpha 2-macroglobulin-induced cellular proliferation. J Biol Chem 277: 36509-36520; Misra U K, Pizzo S V (1998) Ligation of the α2M signaling receptor with receptor-recognized forms of α2-macroglobulin initiates protein and DNA synthesis in macrophages: The effect of intracellular calcium. BiochimBiophysActa 1401: 121-128, Misra U K, Gonzalez-Gronow M, Gawdi G, Hart J P, Johnson C E, et al. (2002) The role of Grp 78 in alpha 2-macroglobulin-induced signal transduction. Evidence from RNA interference that the low density lipoprotein receptor-related protein is associated with, but not necessary for, GRP 78-mediated signal transduction. J Biol Chem 277: 42082-42087; Misra U K, Pizzo S V (2002) Regulation of cytosolic phospholipase A2 activity in macrophages stimulated with receptor-recognized forms of alpha 2-macroglobulin: role in mitogenesis and cell proliferation. J Biol Chem 277: 4069-4078; Misra U K, Gonzalez-Gronow M, Gawdi G, Pizzo S V (2005) The role of MTJ-1 in cell surface translocation of GRP78, a receptor for alpha 2-macroglobulin-dependent signaling. J Immunol 174: 2092-2097). These LRP-1 ligands, however, signal via a pertussis-sensitive G protein and the exact mechanism of signal induction is unknown.
Kupffer cells are strategically positioned in the sinusoidal lumen and play an important role in the removal of altered self or foreign substances from the blood (Kuiper J, Brouwer A, Knook D L, Berkel T J Cv (1994) Kupffer and sinusoidal endothelial cells. In: Arias I M, Boyer J L, Fausto N, Jakoby W B, Schachter D A et al., editors. The Liver: Biology and Pathobiology. 3 ed. New York: Raven Press, Ltd. pp. 791-818; Gumucio J J, Bilir B M, Moseley R H, Berkowitz C M (1994) The biology of the liver cell plate. In: Arias I M, Boyer J L, Fausto N, Jakoby W B, Schachter D A et al., editors. The Liver: Biology and Pathobiology. 3 ed. New York: Raven Press. pp. 1143-1163). Phagocytosis results in the generation of reactive oxygen species (ROS), which are lethal for many microorganisms (Mauël J (1996) Intracellular survival of protozoan parasites with special reference to Leishmania spp., Toxoplasma gondii and Trypanosoma cruzi. AdvParasitol 38: 1-5). The induction of the formation of reactive oxygen species (ROS) is a complex event that requires the assembly of the heterohexameric NADPH oxidase (Brandes R P, Kreuzer J (2005) Vascular NADPH oxidases: molecular mechanisms of activation. Cardiovasc Res 65: 16-27; Groemping Y, Rittinger K (2005) Activation and assembly of the NADPH oxidase: a structural perspective. Biochem J 386: 401-416; Vignais P V (2002) The superoxide-generating NADPH oxidase: structural aspects and activation mechanism. Cell Mol Life Sci 59: 1428-1459; Lambeth J D (2004) NOX enzymes and the biology of reactive oxygen. Nat Rev Immunol 4: 181-189; Quinn M T, Gauss K A (2004) Structure and regulation of the neutrophil respiratory burst oxidase: comparison with nonphagocyte oxidases. J Leukoc Biol 76: 760-781). Kupffer cells represent more than 80% of the total population of tissue macrophages of the body (Kuiper J, Brouwer A, Knook D L, Berkel T J Cv (1994) Kupffer and sinusoidal endothelial cells. In: Arias I M, Boyer J L, Fausto N, Jakoby W B, Schachter D A et al., editors. The Liver: Biology and Pathobiology. 3 ed. New York: Raven Press, Ltd. pp. 791-818). With their strategic position in the sinusoidal lumen, they play an important role in the removal of altered self or foreign substances as well as pathogenic microorganisms from the blood (Kuiper J, Brouwer A, Knook D L, Berkel T J Cv (1994) Kupffer and sinusoidal endothelial cells. In: Arias I M, Boyer J L, Fausto N, Jakoby W B, Schachter D A et al., editors. The Liver: Biology and Pathobiology. 3 ed. New York: Raven Press, Ltd. pp. 791-818; Gumucio J J, Bilir B M, Moseley R H, Berkowitz C M (1994) The biology of the liver cell plate. In: Arias I M, Boyer J L, Fausto N, Jakoby W B, Schachter D A et al., editors. The Liver: Biology and Pathobiology. 3 ed. New York: Raven Press. pp. 1143-1163). Phagocytosis results in the generation of ROS, which are lethal for many microorganisms (Mauël J (1996) Intracellular survival of protozoan parasites with special reference to Leishmania spp., Toxoplasma gondii and Trypanosoma cruzi. Adv Parasitol 38: 1-51). Surprisingly, however, Kupffer cells and other macrophages do not kill viable malaria sporozoites, even after prolonged co-incubation in vitro (Vanderberg J P, Chew S, Stewart M J (1990) Plasmodium sporozoite interactions with macrophages in vitro: a videomicroscopic analysis. J Protozool 37: 528-536; Pradel G, Frevert U (2001) Plasmodium sporozoites actively enter and pass through Kupffer cells prior to hepatocyte invasion. Hepatology 33: 1154-1165., Danforth H D, Aikawa M, Cochrane A H, Nussenzweig R S (1980) Sporozoites of mammalian malaria: attachment to, interiorization and fate within macrophages. J Protozool 27: 193-202; Smith J E, Alexander J (1986) Evasion of macrophage microbicidal mechanisms by mature sporozoites of Plasmodium yoelii yoelii. Parasitology 93: 33-38).
The present invention has addressed the observations noted above and has led the inventors of the present application to hypothesize that the parasites are able to prevent Kupffer cell activation by inducing intracellular signaling events that suppress macrophage activation. Moreover, the present invention demonstrates that by raising the intracellular cAMP concentration in a process that involves syndecan binding and LRP-1 ligation, malaria sporozoites suppress the respiratory burst in Kupffer cells. The present invention proposes the use of this finding for developing novel strategies and therapeutics for treating inflammatory conditions in which reactive oxygen species may play a role in the disease process or may play a role in developing the symptoms or sequelae associated with the disease.